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How to Test Fuel Injectors—3 Simple Methods

Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.

Automotive fuel injectors.

Automotive fuel injectors.

How to Check Fuel Injectors

Testing fuel injectors is sometimes easier than you think, whether on a TBI (throttle body injection) or an EFI (electronic fuel injection) system.

Often, these injectors face common problems you can diagnose visually (TBI), or with the use of some simple tools.

The modern fuel injection system works more efficiently and reliably than the previous carburetor model. The system has better control over the air-fuel mixture under any driving conditions. Occasionally, though, buildup from impurities in the fuel, or injector system electrical or mechanical problems may contribute to rough idle, misfires, harmful emissions, poor engine performance, and fuel economy.

This guide can help you get those injectors back in shape by applying some simple tests you can use for both TBI and EFI systems.


Testing TBI Injectors

- Checking Fuel Spray

Testing EFI Injectors

How to Test a Dead Injector

- Checking the Injector Coil

- Checking the Injector Controlling Circuit

Using a Noid Light

Quiz - Test Your Knowledge of EFI Systems

Testing TBI Injectors

You can visually inspect the fuel spray on TBI system injectors. Fuel flow spray is readily accessible, making it fast to check for potential problems.

Checking Fuel Spray

  1. Remove the cover of the air cleaner housing (where the air filter lives).
  2. Ask an assistant to start (or crank the engine, if it doesn't start).
  3. Check the spray pattern coming out of the injector.

* Fuel should come out partially atomized in an inverted V pattern. A single, solid spray or irregular pattern means the injector needs cleaning, or that an internal part wore out or broke.

You can try to fix an irregular fuel spray pattern by adding a quality cleaning fuel system additive to the fuel tank. Or take your car to the service shop and have them clean the system.

* On the other hand, if you don't see fuel coming out, there may be several reasons for this:

  • Blocked fuel injector
  • Bad injector
  • Injector not receiving power
  • Bad fuel pressure regulator
  • Fuel filter clogged
  • Bad fuel pump

To troubleshoot the injector, apply the tests described in the following section Testing EFI Injectors.

Electronic Fuel Injection system components.

Electronic Fuel Injection system components.

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Testing EFI Injectors

Unlike injectors on throttle body fuel injection (TBI) systems, those used on an Electronic Fuel Injection (EFI) configuration don't have their fuel spraying accessible for inspection. And sometimes the fuel rail assembly, which holds the injectors in place, offers little room to access the injectors without removing the complete assembly. So it's hard to check the spraying pattern of each injector without the proper equipment.

However, you can use a couple of tests that can reveal whether one or more injectors are working on your EFI system and whether the fault is within the injector itself or the controlling circuit.

For this test, you'll listen to each injector to determine whether they are working. As the car computer energizes and de-energizes the injector, the valve inside the injector produces a clicking sound as it opens and closes.

  1. To listen to the injector, you can use a mechanic's stethoscope, an inexpensive tool you can buy at most auto parts stores. However, a long standard screwdriver or even a piece of thin hose of the appropriate length will work just as well.
  2. Start the engine and let it idle.
  3. Apply the parking brake and open the hood.
  4. Place the earpiece of the stethoscope on your ears and the tip of the tool against the side of the injector. If you decided to use a long screwdriver, place the tip of the screwdriver against the body of the injector and the end of the tool's handle against your ear.
  5. When the injector opens and closes, you can hear a clicking sound. The sound comes from the solenoid inside the injector activating and deactivating the injector's valve. If you don't hear the clicking sound, either the solenoid has failed or the computer isn't sending the pulse signal. So you have a dead injector.
  6. Repeat this test on each injector and take note of dead injectors, so that you can test them in the following section.

How to Test a Dead Injector

One of the tests you can do on a dead injector is by checking the injector's coil. For this test, you'll need a multimeter and the resistance value for the coil inside the injector. You can look up the injector resistance value in the service manual for your particular vehicle make and model. If necessary, buy an inexpensive, aftermarket repair manual at your local auto parts store or online. And if you haven't used a multimeter before, check the next video for a quick review on how to operate a multimeter.

Checking the Injector Coil

  1. With the engine off, unplug the electrical connector of the injector you need to test.
  2. Set your digital multimeter to an appropriate value on the Ohms scale according to the resistance specifications for your particular fuel injector (usually, you need to set the multimeter to read at least up to 30 Ohms).
  3. Probe the injector electrical terminals—polarity doesn't matter.
  4. A resistance reading other than the specified in your service manual means you need to replace the injector. For example:
  • If your meter reads infinite resistance, it means the coil in the injector is opened.
  • If your reading is jumping all over the place, the coil is partially opened.
  • however, if you read zero resistance, the coil is shorted.

Checking the Injector Controlling Circuit

You can check for power and pulse signal as it comes from the computer on each dead injector using a test light, an inexpensive and efficient tool.

  1. First, hook the test-light clip to a bolt or bare metal bracket on the engine.
  2. Unplug the electrical connector from the fuel injector you want to test.
  3. Turn the ignition switch to the ON position.
  4. Touch the terminals (one at a time) of the harness connector with the test light. One of the terminals should make the test light glow, this is the injector power source coming from the computer. If the test light doesn't glow, you've found the problem. Check the power side of the circuit for a short, blown fuse, or bad connection along the circuit, including the computer.
  5. Now, plug back the harness connector to the fuel injector and hook the test light clip to the positive side of the battery.
  6. Have an assistant crank or start the engine.
  7. Back probe the opposite wire on the fuel injector connector (this is the pulse signal coming from the computer). If you can't back probe the wire, insert a pin through the wire and use the pin to probe the wire.
  8. This time, the test light should flash, meaning the injector is receiving the pulse signal from the computer to open and close the injector.

* If this is a dead injector and the power and the pulse signals are present, replace the injector.

* If the test light remains lit, the device driver in the computer may have failed or there's a problem in the circuit.

Check your repair manual for further tests, if necessary.

Interpreting Results

If any of the previous tests came out negative, it doesn't necessarily mean your injectors are operating correctly. You tested for some common problems you can troubleshoot at home, but one or more injectors may have a worn or dirty (less common) valve, or a weak or broken return spring that's causing the injector to block or leak fuel. Some of these problems can be a challenge to diagnose without the right tools. But a repair auto shop with professional equipment can help you pinpoint these types of problems.

Test Your Knowledge of EFI Systems

For each question, choose the best answer. The answer key is below.

  1. A fuel injector is basically a solenoid operated valve
    • True
    • False

Answer Key

  1. True

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question: How would you know whether a fuel injector is operating or not?

Answer: The easiest way is to listen to the injector with a mechanic's stethoscope or a length of hose. You should hear a clicking sound as the injector's valve opens and closes. However, if the injector is clogged, even if it's working it'll not spray any fuel. You can also use an ohmmeter to test for an open or shorted coil inside the fuel injector.

Question: What causes inconsistent fuel injector noid readings in different cylinders?

Answer: This could be a problem with the injector drivers, a loose wire in the circuit or problem with the computer itself.

Question: what do you repair or replace if there is no pulse on the injectors when every other thing is okay?

Answer: Check the connection at the ECM. If none of the injectors are receiving the signal, there could be the problem. Look up the electric diagram for your model. If you don't have the manual, your local library may have a copy.

Question: What is the cause of an inoperative fuel pump?

Answer: Make sure the pump is receiving adequate voltage. Check for fuel pressure and (if possible) volume. Also, it is possible the pump is worn out and in need of replacement, if voltage/current checks out OK.

Question: What is a fuel pressure test?

Answer: This other post gives you the idea about pressure and how to go about the test:

Question: Do TBI injectors have to be placed facing a certain way?

Answer: Yes, the mounting base will pretty much set it back in place.

Question: Audi 2.0 TDI. It idles and revs near perfect. While driving, it is perfect until it starts misfiring. I turn the ignition off/on and perfect again for a few km. This can happen under load and under no load like coasting or downhill. When it is going, she pulls like a train. What could be wrong?

Answer: Download the trouble code from the computer. If you don't have a scanner, most auto parts store will get the codes for you. Usually, this is due to a bad spark plug wire, coil or fuel injector. If the misfire is reported on cylinder 1, for example, swap wires or coil with other cylinder and see if the misfire moves as well. You may need to test the injector.

Question: How do you test a badly worn needle and nozzle body of an injector?

Answer: Removing the sensor and watching its operation while out of the fuel rail can give you some clues about fuel flow and atomization. However, this is possible in some models and much harder to do in other models. See if you can pull the injector you want to check and watch the sensor while cranking the engine for a few seconds. You have to be careful while doing this test and not drop anything thought the injector hole while it is out.

Question: Why on a TBI am I getting gas into the MAP sensor vacuum line?

Answer: Hard to tell, but it can happen--depending on the model configuration--when the fuel pressure regulator diaphragm goes bad and starts leaking fuel into the throttle-intake; or another source of a leak that is "flooding" the throttle. Check the other hoses connected to the intake and see if you can find a trace of gas. Hope I could be of more help.

Question: Can a fuel injector fail on high fuel pressure only? I am having a misfire only on wide open throttle.

Answer: A slightly dirty fuel injector may work fine at low speeds, but prevent enough fuel delivery at high speed. Usually, you'll get a lean code (P0171 or P0174). With a misfire, you may get other codes caused by a bad injector (P0204), EGR system. If you are getting only a code for a possible misfire, check ignition system components for the affected cylinder. This other post may help as well:

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